Recently, it is expected that conventional energy sources such as oil or charcoal will be exhausted, and thus interests in alternative energy are increasing. As one of the alternative energy, a fuel cell has high efficiency, does not emit a pollutional material such as NOx or SOx, and is supplied with an abundant fuel, and thus it is the center of attention.
The fuel cell is an electrical power system for converting a chemical reaction energy of a fuel and an oxidant into an electrical energy. Typically, hydrocarbon such as hydrogen, methanol or butane is used as a fuel, and oxygen is used as an oxidant.
The fuel cell includes a polymer electrolyte membrane fuel cell (PEMFC), a direct methanol fuel cell (DMFC), a phosphoric acid fuel cell (PAFC), an alkaline fuel cell (AFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC) and so on. Among them, the PEMFC has good energy density and high output, and thus its research and development is made briskly. The PEMFC is different from the other fuel cells in that a solid polymer electrolyte membrane is used, but not a liquid electrolyte.
For commercialization, the essential considerations of the PEMFC are currently performance improvement, life prolongation and competitive price. These three factors are affected most by a membrane electrode assembly (MEA), and the present invention relates to the MEA, in particular, an ion exchange membrane of the MEA.
In operation of a fuel cell, effective management of water is one of the most important factors that affect the performance of the fuel cell. In practice, a wet fuel is fed into a fuel cell to prevent dryness of the membrane electrode assembly. Water generated at a cathode should be promptly removed not to obstruct the fuel supply.
Note that water is important particularly in the membrane electrode assembly. When water supply is insufficient, an ion exchange membrane has a sudden reduction in conductivity of hydrogen ions. Consequently, performance of the membrane electrode assembly deteriorates in a short period, and in a long period, partial or total deterioration of the ion exchange membrane is promoted to create a pinhole of the membrane, which results in expiration of the life of the membrane electrode assembly. Therefore, to maximize the performance and life of the membrane electrode assembly, the ion exchange membrane should be supplied/maintained with the required optimum water.
Meanwhile, an excessive amount of water results in dissolution of a catalyst, oxidation acceleration of a gas diffusion layer of a membrane electrode assembly, deterioration of operational stability, or enlargement or addition of external devices for humidifying. Therefore, it requires an ion exchange membrane capable of ensuring sufficient ion conductivity at low RH of 50% or lower. Such a low humidity ion exchange membrane needs a small amount of absolute water, and thus, has advantages of stable operation and management of a system at ease, removal or reduction of components related to water management, and even in case of low relative humidity, operation at high temperature of 80° C. or higher.
Conventionally, in the case of a perfluorosulfonic acid membrane that is a fluoride-based ion exchange membrane, attempts have been made to lower EW (Equivalent Weight) indicating an amount of SO3H group that performs a hydrogen ion conducting function, i.e. to increase ion conductivity, thereby leading to high ion conductivity at a given relative humidity. In the case of a hydrocarbon-based ion exchange membrane that is used in place of the fluoride-based ion exchange membrane, it is known that the hydrocarbon-based ion exchange membrane should have lower EW than the fluoride-based ion exchange membrane to obtain the same ion conductivity as the fluoride-based ion exchange membrane.
In other words, high ion conductivity results in low EW, which means a large amount of hydrogen ion conducting SO3H group per unit gram of an ion exchange membrane.
However, as an amount of SO3H group increases, polar solvent (water or alcohols) stability of an ion exchange membrane falls down rapidly, dimensional stability fails under wet operating conditions of a fuel cell, and finally the membrane dissolves. To solve the problem, the present invention provides a configuration to ensure mechanical and chemical stability using an ion exchange membrane made from an ion conductive material having a maximum amount of SO3H for maximum ion conductivity.